The world supply of fossil fuel for the production of energy is being exhausted at an ever increasing rate. This has resulted in a continuing energy and economic crisis which impacts not only on the world's economy but on peace and stability. Solutions to the energy crisis include the development of new fuels and the development of more efficient technologies to utilize existing fuels. One method of more efficiently utilizing existing fuels, including energy conservation, power generation, environmental protection, and economic growth, is the thermoelectric generation of electricity.
In the thermoelectric generation of electricity electrical power is generated by heat. It has been estimated that two-thirds of all energy, for example from automobile exhausts, fossil fuel power plants, and the like is discharged to the environment without further recovery. This so called waste heat is paid for and then discharged into the environment without use. Employment of waste heat for the generation of electricity can provide a direct reduction in thermal polution and an increase in economically efficient utilization of fuels, independent of the original source of the thermal energy.
The performance of a thermoelectric device can be expressed in terms of a figure of merit (Z) for the material forming the device. Z is defined by the relationship: EQU Z=S.sup.2 (K sigma)
where
Z is a dimensionless quantity, PA1 S is the Seebeck coefficient in microvolts per degree centigrade, PA1 K is the thermal conductivity in milliwatts per centimeter per degree centigrade, and sigma is the electrical conductivity in reciprocal (ohm-centimeters).
In order for a material to be suitable for thermoelectric power generation, the thermoelectric power coefficient, that is the Seebeck coefficient, S, must be high, the electrical conductivity, sigma, must be high, and the thermal conductivity, K, must be low.
The thermal conductivity, K, has two components, K.sub.1, the lattice component, and K.sub.e, the electrical component. In non-metals the lattice component, K.sub.1, dominates and it is this component which mainly determines the value of K.
Therefore, in order for material to be efficient for thermoelectric power conversion, charge carriers must diffuse easily through the hot junction to the cold junction while maintaining a temperature gradient between the two junctions. Thus high electrical conductivity is required along with low thermal conductivity.
Historically, thermoelectric power conversion has not found wide commercial usage. The major reason for this has been that thermoelectric materials which were suitable for commercial applications have been crystalline. Those crystalline materials which are best suited for thermoelectric devices have been difficult to manufacture because of poor mechanical properties and extreme sensitivity of material properties to macroscopic compositional changes. This is because prior art crystalline thermoelectric materials contain a predominace of chalcogenide elements, tellurium and selenium. Tellurium and selenium are natural glass formers. It is because of this tendency of tellurium and selenium to form glasses that the growth, control, and mechanical stability of prior art thermoelectric crystalline materials has been substantially non-reproducible.
The chalcogenides, such as tellurium, only grow high quality single crystals with great difficulty. Even when tellurium containing single crystals are grown, the crystalline materials are unstable materials with large defect densities, and compositions far from stoichiometric. For these reasons, controlled doping has proven to be extremely difficult.
Moreover, crystalline solids have been unable to attain large values of electrical conductivity and, simultaneously, low thermal conductivity.
The polycrystalline thermoelectric materials are (Bi, Sb).sub.2 (Se,Te).sub.3, PbTe, and Si-Ge. The bismuth-antimony tellurides represent a continuous solid system in which the relative amount of bismuth and antimony are from 0 to 100%. Polycrystalline materials also present problems in that the polycrystalline materials have polycrystalline grain boundaries, resulting in relatively low electrical conductivities. Moreover, fabrication of polycrystalline thermoelectric materials into suitable thermoelectric devices have presented difficulties.
Improved thermoelectric materials have been developed which are not single phase crystalline materials, but are instead, disordered materials. These materials, more fully disclosed in copending U.S. application Ser. No. 341,864 filed Jan. 22, 1982 in the names of T. J. Jayadev and On Van Nguyen for New Multiphase Thermoelectric Alloys and Methods Of Making The Same now abandoned and replaced by U.S. Ser. No. 412,306 filed Aug. 27, 1982, now U.S. Pat. No. 4,447,277, issued May 8, 1984 incorporated herein by reference. The materials of Jayadev and Nguyen are multiphase materials having both amorphous and multiple crystalline phases. These materials are good thermal insulators, and include grain boundaries of various transitional phases varying in composition from the composition of matrix crystallites to the composition of the various phases in the grain boundary region. The grain boundaries are highly disordered with the transitional phases including phases of high thermal resistivity to provide high resistance to thermal conduction. The materials of Jayadev and Nguyen have grain boundaries defining regions which include conductive phases therein, providing numerous electrical conduction paths through the bulk material for increasing electrical conductivity without substantially affecting thermal conductivity. In essence, the materials have all the advantages of polycrystalline material, with desirably low conductivities and crystalline bulk Seebeck properties. Moreover, the disordered multiphase materials also have high electrical conductivity. Thus, the materials of Jayadev and Nguyen have an S.sup.2 (sigma) product for the figure of merit which can be independently maximized with desirably low thermal conductivities for thermoelectric power generation.
The materials of Jayadev and Nguyen are fabricated in a manner which introduces disorder into the material on a macroscopic level. This disorder allows various phases, including conductive phases, to be introduced into the materials.
Commonly assigned copending U.S. application Ser. No. 414,917 filed Sept. 3, 1982 by T. J. Jayadev, On Van Nguyen, Jaime M. Reyes, H. Davis, and M. W. Putty, (hereinafter "Jayadev et al") now U.S. Pat. No. 4,588,520 incorporated herein by reference, describes compacted and/or compressed powder materials useful for thermoelectric applications. The powdered materials have compositional disorder, translational disorder, configurational disorder, and other disorders introduced therein. The powder materials are multiphase alloy materials having a first phase, including matrix crystallites bounded by disordered grain boundaries at various phases including transitional phases. Between the grain boundaries are macroscopic grain boundary regions which also include various phases, including electrically conductive phases and crystalline inclusions. The grain boundary regions are rich in electrically conducting modifying phases which provide high electrical conductivities. The other phases in the grain boundary regions and the grain boundaries provide low thermal conductivities.
The compacted materials further include additional bulk disorder between the interfaces of the compacted powder particles which further reduce thermal conductivity. The materials comprise a body formed from compacted powder material. The compacted material includes bismuth, tellurium, and at least one highly electrically conductive phase.
The materials described in Jayadev et al. are made by forming a mixture containing the constituent elements of a first compound including at least bismuth and tellurium and constituent elements of a second compound capable of forming at least one highly electrically conductive phase, and thereafter compressing at least a portion of the particulate mixture to form a compacted body of the material. The first and second compounds may be separately prepared from the respective constituent elements, and then the first and second compounds combined and heated to form a melt, with the melt cooled, to form a solid material form which is crushed to form the particulate material.
Alternately, a melt may be formed from the second compound and the constituent elements of the first compound and then cooled, for example by planar flow casting, to a solid material form and crushed to form the particulate mixture. According to a further alternative, the first and second compounds, that is the bismuth and tellurium compound, and the compound capable of forming at least one highly electrically conductive phase, may be separately prepared from their respective constituent elements and separately crushed into particulate form to form the particulate mixture.
The first compound includes bismuth, antimony, and tellurium for making a p-type material and bismuth, tellurium, and selenium for making an n-type material. The second compound, to be combined with either of the first compounds, that is, with either the p-type material or the n-type material, includes silver, antimony, and tellurium.
Individual thermoelectric elements of the n-type drive negative carriers from the hot side of the device to the cold side of the device, while individual thermoelectric elements of the p-type conductivity drive positive carriers from the hot side of the device to the cold side of the device. Operative thermoelectric devices are characterized by a plurality of thermoelectric elements, thermally in parallel and electrically in series. N-type elements and p-type elements of the thermoelectric device are assembled so that they are thermally in parallel and electrically in series with one another. Each pair of elements contain one n-type thermoelectric element and one p-type thermoelectric element electrically connected at one end by an electrical connecting strap. Each strap connects the end of an n-type element of each pair of electrically connected thermoelectric elements to the p-type element of the next adjacent pair of electrically connected thermoelectric elements. Thus, all of the individual n-type and p-type thermoelectric elements of a thermoelectric device are connected electrically in series and thermally in parallel. In order to achieve maximum power output for a thermoelectric device, the electrical resistance of both the individual strap and of the thermoelectric device to strap contact must be minimized.
The preferred solders for temperatures of intended use are solders containing, inter alia, one or more of gold, silver, lead and tin. The materials of this solder as well as the high concentration of tellurium in the individual thermoelectric body gives rise to the in-migration of solder material, for example tin, and/or the out-migration of elements, for example, selenium, tellurium, and antimony from the thermoelectric body. The effects of either or both of the in-migration of solder material to the thermoelectric body, and the out-migration of the elements from the thermoelectric body, attacks and weakens the solder joint between the thermoelectric body and the straps. This is initially evidenced by a high contact resistance, a decrease in an apparent figure of merit, Z, for the individual element which is the result of changes in stoichiometry thereof, and, ultimately, by physical failure of the joint.
The migration problem, including one or both of in-migration of solder material to the thermoelectric body and out-migration of elements from the thermoelectric body, is sharply temperature dependent and is a more severe problem on the hot side of a thermoelectric device.
Attempts to solve the problems of migration have typically involved the use of barrier layers between the straps and the thermoelectric elements. However, such barriers have not been totally satisfactory, and diffusion remains a major problem in thermoelectric generators.